Cathode heater for electron discharge device



Jan. 20, 1970 ALMER ET AL 3,490,944

CATHODE HEATER FOR ELECTRON DISCHARGE DEVICF Filed Sept. 9, 1966 addstive materal mixed with aluminum oxecie coating aluminum oxide coating filament wire INVENTORS FRIEFJERICH am. ALMER ADREANUS KUIPER BY CORNELBS MAAN .7 GENT United States Patent 3,490,944 CATHODE HEATER FOR ELECTRON DISCHARGE DEVICE Friedrich Hermann Raymund Almer, Adrianus Kuiper,

and Cornelis Maan, Emmasingel, Eindhoven, Netherlands, assignors, by mesne assignments, to U.S. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Sept. 9, 1966, Ser. No. 578,143 Claims priority, applicatgrizlgztgherlands, Sept. 23, 1965,

Int. Cl. C231) /50 U.S. Cl. 117217 2 Claims ABSTRACT OF THE DISCLOSURE A heater for an indirectly heated cathode is coated with a layer of alumina (A1 0 and with a dark colored layer consisting of metal oxide particles such as magnesia, beryllium oxide, thorium oxide, or zirconium oxide contain ing darker colored metal particles, i.e., tungsten.

The invention relates to a heater body for use in an indirectly heated cathode, this body being coated with a layer preferably consisting of pure alumina and with a dark-colored layer consisting of a mixture of metal oxide particles and metal particles of darker color, and more particularly to a heater body in which the dark-colored particles consist of tungsten.

It is known that a heating wire can be coated with an insulating layer consisting of pure alumina to which a second layer is applied which consists of alumina mixed with approximately 50% by weight of tungsten powder. Thus, the heat radiation towards the cathode is increased so that the temperature of the heating wire is reduced because the temperature difference between the heater body and the cathode is smaller. Consequently, the insulation resistance of the alumina layer is also higher, while the life of the heating wire is considerably lengthened.

Although satisfactory results are obtained with this known embodiment, it has been found that during manufacture the percentage of rejects is likely to be comparatively high. It is found that the dark-colored layer has a considerably lower insulation resistance than the subjacent A1 0 layer. This may result in the occurrence of leakage current between the cathode and the heater body or of breakdown if the A1 0 substratum exhibits pores or cracks. The substratum must therefore be sintered very carefully, which is expensive. Moreover, the dark-colored layer must terminate at a certain distance from the end of the subjacent A1 0 layer in order not to get into contact with the heating wire.

It has been found that much more favorable results can be obtained if according to the invention the metal oxide particles of the dark-colored layer consist of one or more of the following oxides: magnesia, beryllium oxide, thorium oxide, zirconium oxide. Thorium oxide and zirconium oxide have proved to be particularly suitable and it has been found that with the use of these Oxides the percentage of rejects is low. The insulation resistance between the cathode and the heating wire is also found to be particularly high during the cathode lifetime.

Although this has not been proved with certainty, the high resistance may be due to the possibility that these oxides react with the A1 0 of the substratum and form a junction layer so that the p-n junctions may be obtained which consequently have a blocking effect on leakage currents.

It is known that in this manner a very high insulation resistance can be obtained between the heating Wire and the cathode because the A1 0 of the first layer forms with the tungsten wire an n-type junction layer, whereas the pure A1 0 itself is of the p-conductivity type, just like MgO. However, MgO forms with A1 0 likewise a p-type interface. Therefore, in the known construction, the A1 0 layer on the heating wire is in contact with a layer of MgO which results in the formation of pup-junctions from the heating wire to the cathode so that leakage currents are blocked in both directions.

The possibility of using in practice ZrO and ThO as additions to the dark-colored layer is due to the surprising fact that these oxides, which in their pure state can be dissolved in nitric acid, after being mixed with approximately 40 to by weight of tungsten powder, are found to be no longer attacked by the mixture of nitric acid and sulfuric acid used for dissolving the molybdenum core on which the heating wire is wound. Moreover, it has been found that a sintering temperature of 1600 C. is sufiicient to obtain a satisfactorily adhering dark-colored layer, while the sintering temperature of these oxides in the pure state is considerably higher.

Although it is not necessary, it has moreover been found to be of advantage if the mean size both of the oxide particles and of the metal particles of the dark-colored layer is approximately the same and is preferably approximately ln while the largest particles are preferably not larger than 12,. This can be obtained by sieving the powder ground for a long time in a sieve having 360 meshes/cm. As described in copending application Ser. No. 575,570, filed Aug. 29, 1966, now abandoned. It has been found that the insulation resistance may be further increased by the application to the dark-colored layer of a third layer consisting or one or more of the oxides of Mg, Be, Zr, or Th. If this layer has a thickness of only a few microns, the heat radiation of the dark-colored layer is substantially not influenced, since such a thin layer is pervious to the heat radiation.

The drawing illustrates the coated article of the invention.

While the invention has been described in connection with specific examples and applications, other modifications will be apparent to those skilled in the art without departing from the spirit and scope thereof as defined in the appended claims.

What is claimed is:

1. A heater body for use in an indirectly heated cathode, said body being coated with a layer consisting of pure alumina and with a dark-colored layer consisting of a mixture of metal oxide particles and metal particles of darker color, said metal oxide particles consist of an oxide selected from the group consisting of magnesia, beryllium oxide, thorium oxide and zirconium oxide.

2. A heater body as claimed in claim 1, in which the metal particles of the dark-colored layer consist of tungsten.

References Cited UNITED STATES PATENTS 3,195,004 7/1965 Hassett 313340 3,328,201 6/1967 Scheible 117215 WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R. 

